Effect of Power Cooling Water to the Ecosystem of Youngil Bay and Hyeongsan River Estuary in South Korea

Abstract

The POSCO (Pohang Steel Company) discharges about 1,000,000 m(3)/day of 30 degrees C power cooling water to the estuary of Hyeongsan River, which is located in the southeastern region of South Korea. The thermal effluent is mixed with sea water and diffused to Youngil Bay. At the beginning of the 1990s, the thermal effluent affects brackish water zone of Hyeongsan River and Youngil Bay and becomes a main cause of red tide in this sea area, which starts to occur from January. Besides, the thermal effluent also affects coastal sea water quality and ecosystem raising the surrounding sea water temperature. In this study, we figured out the present circumstances and problems in the brackish water zone of Hyeongsan River estuary and Youngil Bay, which resulted from the thermal effluent in winter season. We also found out the cause of red tide on the basis of 8 measured categories on site, such as temperature, salinity, DO, COD, TN, TP, chlorophyll-a, and flow velocity, and performed a numerical analysis using the measured data to precisely investigate the occurrence mechanism of red tide. The results show that intrusion range of saline wedge in the lower layer of water depth is reduced in summer season because of increased river discharge, however, the river water and thermal effluent is diffused to Youngil Bay forming a typical estuary front with stratified condition. On the contrary, in winter season, a constant vertical density distribution, resulting from a strong mixing of the upper and lower water layers, is formed at downstream of the thermal effluent outlet. However, a thermohaline front, developing a less denser distribution than that developed in the strong mixing area, is formed at downstream of the strong mixing area. The formation of thermohaline front induces flow stagnation in Youngil Bay and subsequently obstructs river discharge to Youngil Bay. Thus, a mass of chlorophyll-a is bred by the thermal effluent and consequently a red tide.